The filtration efficiency of ventilation air cleaners is highly particle-size dependent over the 0.01 to 3 mu m diameter size range. Current standardized test methods, which determine only overall efficiencies for ambient aerosol or other test aerosols, provide data of limited utility. Because particles in this range are respirable and can remain airborne for prolonged time periods, measurement of air cleaner fractional efficiency is required for application to indoor air quality issues. The objectives of this work have been to 1) develop a test apparatus and procedure to quantify the the fractional filtration efficiency of air cleaners over the 0.01 to 3 mu m diameter size range and 2) quantify the fractional efficiency of several induct air cleaners typical of those used in residential and office ventilation systems. Results show that efficiency is highly dependent on particle size, flow rate, and dust load present on the air cleaner. A minimum in efficiency was often observed in the 0.1 to 0.5 mu m diameter size range. The presence of a dust load frequently increased an air cleaner's efficiency; however, some air cleaners showed little change or a decrease in efficiency with dust loading. The common furnace filter had fractional efficiency values of less than 10% over much of the measurement size range

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